Conventionally, as an ignition timing control system for an engine, one disclosed in Patent Literature 1 is known. This engine is of a direct fuel-injection type which is configured to directly inject fuel into cylinders, and in the engine, homogeneous combustion in which fuel is injected during the suction stroke and a homogeneous air-fuel mixture generated in a combustion chamber is burned and stratified combustion in which fuel is injected during the compression stroke and a stratified air-fuel mixture generated concentrically in the vicinity of an ignition plug is burned are selectively performed depending on operating conditions of the engine.
Further, in this ignition timing control system, ignition timing for the stratified combustion is set according to an engine speed and a basic fuel injection amount (or target engine torque). The basic fuel injection amount is set according to the engine speed and an intake air amount, and the target engine torque is determined based on an accelerator pedal opening and a vehicle speed, and is further determined according to a transmission gear ratio and a torque ratio.
In the stratified combustion, combustion is performed by igniting the stratified air-fuel mixture generated in the vicinity of the ignition plug by the ignition plug, and hence in order to obtain stable ignition and combustion, it is required to perform ignition in such a proper timing that a rich air-fuel mixture exists in the vicinity of the ignition plug. However, in the conventional ignition timing control system, ignition timing for the stratified combustion is set according to only the engine speed and the basic fuel injection amount or the target engine torque, and hence it is not necessarily possible to properly set the ignition timing, depending on the operating conditions of the engine. As a result, a misfire or knocking is caused, making it possible to obtain stable combustion, and hence there is a fear that drivability is lowered.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide an ignition timing control system for an engine, which is capable of ensuring stable combustion without causing a misfire or knocking, by properly setting ignition timing in the compression stroke injection mode, thereby making it possible to improve drivability.
[Patent Literature 1]
Japanese Laid-Open Patent Publication (Kokai) No. H10-331752.